U.S. patent number 4,836,209 [Application Number 07/193,212] was granted by the patent office on 1989-06-06 for nmr imaging of moving material using variable spatially selected excitation.
This patent grant is currently assigned to Stanford University. Invention is credited to Dwight G. Nishimura.
United States Patent |
4,836,209 |
Nishimura |
June 6, 1989 |
NMR imaging of moving material using variable spatially selected
excitation
Abstract
A method of imaging material flowing through a slab in a body
using magnetic resonance techniques includes placing the body in a
magnetic field including a first magnetic (z) gradient for slab
thickness selection. A first rf pulse (180.degree.) is applied to
the body at a frequency band and of sufficient strength to flip
nuclear spins located essentially in the slab. After allowing
moving material in the slab to flow from the slab, a second rf
(90.degree.) pulse is applied to the body at a frequency band of
sufficient strength to flip the nuclear spins in the slab for
generating a signal. First image date in an X-Z plane is obtained
from the nuclear spins flipped by the second 90.degree. rf pulse.
Thereafter, a third 180.degree. rf pulse is applied to the body. In
one embodiment the third rf pulse is non-selective and is of
sufficient strength to flip nuclear spins in the body including but
not limited to the slab. Moving material is again allowed to flow
from the slab, and a fourth 90.degree. rf pulse is then applied to
the body at a frequency band and of sufficient strength to flip
nuclear spins in the slab. Second image data in an X-Z plane is
obtained from nuclear spins flipped by the fourth 90.degree. rf
pulse. The first image data is subtracted from the second image
data to obtain third image data of moving material through the
slab. Alternatively, the first image data and the second image data
can be gated to different portions of a cardiac cycle whereby the
flow of moving material differs in obtaining the first image data
from the flow of moving material in obtaining the second image
data. Direction sensitivity can be obtained by selective spatial
excitation of portions of the body.
Inventors: |
Nishimura; Dwight G. (Palo
Alto, CA) |
Assignee: |
Stanford University (Stanford,
CA)
|
Family
ID: |
26888784 |
Appl.
No.: |
07/193,212 |
Filed: |
May 11, 1988 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
894319 |
Aug 7, 1986 |
|
|
|
|
Current U.S.
Class: |
600/419; 324/306;
324/309 |
Current CPC
Class: |
G01R
33/563 (20130101); G01R 33/48 (20130101) |
Current International
Class: |
G01R
33/563 (20060101); G01R 33/54 (20060101); G01R
33/48 (20060101); A61B 005/05 () |
Field of
Search: |
;128/653
;324/306,309 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Ruth S.
Attorney, Agent or Firm: Flehr, Hohbach, Test, Albritton
& Herbert
Parent Case Text
This is a continuation of application Ser. No. 894,319 filed Aug.
7, 1986, abondoned.
Claims
What is claimed is:
1. A method of projection imaging moving material in a
three-dimensional slab through a body using magnetic resonance
techiniques comprising the steps of
(a) placing said body in a magnetic field including a first
magnetic gradient (z) for slab selection,
(b) applying a first rf pulse to said body at a frequency band and
of sufficient strength to flip by a first angle nuclear spins
limited essentially to said slab,
(c) allowing moving material outside of said slab to flow into said
slab,
(d) applying at least a second rf pulse to said body at a frequency
band and of sufficient strength to flip nuclear spins in said slab
by a second angle suitable for generating a signal, thereby
flipping nuclear spins of moving material that moves into said slab
during step c,
(e) obtaining first projection image data in an X-Z plane from said
nuclesr spins flipped by said second rf pulse,
(f) applying a third rf pulse to said body at a frequency band and
of sufficient strength to flip nuclear spins in said body,
including said slab but not limited to said slab by said first
angle,
(g) repeating step c,
(h) repeating step d with a fourth rf pulse identical to said
second pulse,
(i) obtaining second projection image data in said X-Z plane from
said nuclear spins flipped by said fourth rf pulse,
(j) subtracting said first projection image data from said second
projection image data to obtain a third image data of moving
material moved to said slab, and
(k) imaging said X-Z plane with said third image data.
2. The method as defined by claim 1 wherein step b further includes
flippin nuclear spins on one side of said slab, and step j obtains
third image data of moving material flowing into said slab from a
side opposite from said one side.
3. The method as defined by claim 1 wherein said first angle is
approximately 180.degree. and said second angle is approximately
90.degree..
4. The method as defined by claim 3 wherein said steps (d) and h)
occur when the longitudinal magnetization of excited static
material is approximately zero.
5. The method as defined by claim 1 wherein step (d) includes
applying a 90.degree. rf pulse and a 180.degree. pulse for spin
echo signal detection.
6. The method as defined by claim 1 wherein steps (a) through (k)
are repeated for other slabs to increase the field of view.
7. A method of imaging moving material in a slab through a body
using magnetic resonance techniques comprising the steps of
(a) placing said body in a magnetic field including a first
magnetic gradient (z) for slab selection,
(b) applying a first rf pulst to said body at a frequency band and
of sufficient strength to flip by a first angle nuclear spins
limited to the region on one side of said slab,
(c) allowing moving material in said slab to flow from said slab
and material from said one region to flow into said slab,
(d) applying a second rf pulse to said body at a frequency band and
of sufficient strength to flip nuclear spins in said slab by a
second angle suitable for generating a signal,
(e) obtaining first image data in an X-Z plane from said nuclear
spins flipped by said second rf pulse,
(f) repeating step c without exciting said region,
(h) repeating step d,
(i) obtaining second image data in an X-Z plane in said slab from
said nuclear spins flipped by the step h rf pulse,
(j) subtracting said first image data from said second image data
to obtain a third image data of moving material to said slab,
and
(k) imaging said third image data.
8. The method as defined by claim 7 wherein said step d) includes
applying a 90.degree. rf pulse and a 180.degree. pulse for spin
echo signal detection.
9. The method as defined by claim 7 wherein said first angle is
approximately 180.degree. and said second angle is approximately
90.degree..
10. The method as defined by claim 7 wherein steps (a) through (k)
are repeated for other slabs to increase the field of view.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to nuclear magnetic resonance
(NMR) or magnetic resonance (MR) imaging, and more particularly the
invention relates to the selective projection imaging of moving
material by magnetic gradient manipulation.
Techniques are known for magnetic resonance angiography in which
blood flow is imaged. U.S. Pat. No. 4,528,985 utilizes a temporal
subtraction technique in which image data at two different time
intervals is obtained and subtracted so that data for static
material cancels and data for moving material (e.g., blood)
provides a residual image.
U.S. Pat. No. 4,516,582 excites nuclear spins in a thin slab and
employs a gradient field to dephase excited spins of static
material. After a time interval in which excited nuclear spins of
blood flow from the thin slab, the nuclear spins and the slabs are
again excited. Due to the dephasing to the static spins,
substantially all of the subsequent resulting NMR signal will come
from the excitation of spins of blood flow which moves into the
slab during the time interval.
U.S. Pat. No. 4,647,857
for FLOW MEASUREMENT USING NUCLEAR MAGNETIC RESONANCE utilizes spin
echo techniques to eliminate the effects of static nuclear spins
whereby a residual signal from dynamic or moving nuclear spins is
obtained.
Disclosed in copending application Ser. No. 894,318, filed Aug. 7,
1986, now U.S. Pat. No. 4,718,424 is a method of imaging blood flow
utilizing the selective effects of magnetic gradient field waveform
moments on static material and on flowing material. By varying the
polarity and duration of a magnetic field gradient in which
material is positioned, the magnitude or phase of nuclear spins
signals and FIDs can be varied depending on the motion of the
material in a direction aligned with the magnetic field
gradient.
SUMMARY OF THE INVENTION
An object of the invention is a method of and apparatus for imaging
moving material using variably spatially selected excitations.
Another object of the an invention is an improved method of
subtraction angiography.
A feature of the invention is the use of a first excitation pulse,
180.degree. for example, for inverting nuclear spins in a slab.
After a period of time during which flowing material from outside
the slab flows into the slab, a second excitation pulse, 90.degree.
for example, is applied for generating a first image signal from
the flowing material in the slab. Thereafter, the excitation
sequence is repeated for the slab and for at least one adjacent
region including material flowing into the slab to obtain a second
image signal. The second image signal is subtracted from the first
image signal to remove any residual signal from static material
thereby obtaining a difference signal corresponding to flowing
material. The necessary data acquisition is performed on the
generated signal along the thickness axis of the slab to form a two
dimensional (2-d) image of the volume using spin warp imaging or
other methods such as echo planar or generalized imaging with time
variant gradients.
Since static material has different T.sub.1 values in general,
static material signals may remain in the generated image signals.
To the extent that these remaining signals dominate the blood
signals, they are eliminated by subtracting the signals. This is
readily accomplished in accordance with the invention by acquiring
the second image signal with the same static material component as
in the first image but a different signal component from blood.
The invention and objects and features thereof will be more readily
apparent from the following detailed description and appended
claims when taken with the drawing.
DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1D illustrate the arrangement of NMR apparatus and
magnetic fields generated therein.
FIG. 2 is a functional block diagram of NMR imaging apparatus.
FIG. 3 illustrates a basic pulse sequence for exciting a slab and
the resultant projection image for use in accordance with one
embodiment of the invention.
FIG. 4 is a plot illustrating longitudinal relaxation with time
after inversion using the pulse sequence of FIG. 3.
FIG. 5 is an illustration of a pulse sequence for obtaining a
second image for subtraction imaging in accordance with the
invention.
FIGS. 6A and 6B illustrate top views of slabs and adjoining regions
with the shaded regions indicating regions excited by 180.degree.
pulses in accordance with one embodiment of the invention for
obtaining two image signals.
FIGS. 7A and 7B are top views of slabs and adjoining regions with
the shaded portions indicating regions excited by 180.degree.
pulses in accordance with another embodiment of the invention for
obtaining two image signals.
DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS
Referring now to the drawings, FIG. 1A is a perspective view
partially in section illustrating coil apparatus in NMR imaging
system, and FIGS. 1B-1D illustrate field gradients which can be
produced in the apparatus of FIG. 1A. This apparatus is discussed
by Hinshaw and Lent, "An Introduction to NMR Imaging: From the
Bloch Equation to the Imaging Equation", Proceedings of the IEEE,
Vol. 71, No. 3, March 1983 pgs. 338-350. Briefly, the uniform
static field B.sub.o is generated by the magnet comprising the coil
pair 10. A gradient field G(x) is generated by a complex gradient
coil set which can be wound on the cylinder 12. An RF field B.sub.1
is generated by a saddle coil 14. A patient undergoing imaging
would be positioned along the z axis within the saddle coil 14.
In FIG. 1B an X gradient field is shown which is parallel to the
static field B.sub.o and varies linearly with distance along the X
axis but does not vary with distance along the Y or Z axes. FIGS.
1C and 1D are similar representations of the Y gradient and Z
gradient fields, respectively.
FIG. 2 is a functional block diagram of the imaging apparatus as
disclosed in NMR-A Perspective on Imaging, General Electric Company
1982. A computer 20 is programmed to control the operation of the
NMR apparatus and process FID signals detected therefrom. The
gradient field is energized by a gradient amplifier 22, and the RF
coils for impressing a RF magnetic moment at the Larmor frequency
is controlled by the transmitter 24 and the RF coils 26. After the
selected nuclei have been flipped, the RF coils 26 are employed to
detect the FID signal which is passed to the receiver 28 and thence
through digitizer 30 for processing by computer 20.
In accordance with the present invention, moving material such as
blood flow is imaged by means of a variably spatially selective
excitation. The invention uses time of flight effects to
distinguish signals of flowing blood from signals of static
material. The basic procedure is illustrated in FIG. 3 and includes
the following steps:
1. Selectively excite a slab 11 (perpendicular to the Z axis, for
example) with an inverting pulse 13 (180.degree. or other large
flip angle). This represents a preparatory pulse.
2. Wait TI seconds. This is the evolution period.
3. Selectively excite the same slab 11 with a pulse 15 or pulse
suitable for generating a signal (for example 90.degree. or a
90.degree.-180.degree. combination for spin echo). The RF pulses
are applied in the presence of a Z gradient, G.sub.Z, as
illustrated.
4. With the generated FID signal, the necessary data acquisition is
performed to form a two dimension (X-Z) image of the volume using
spin warp imaging or other methods such as echo planar or
generalized imaging with time varying gradients.
The image resulting from the above procedure will contain a
relatively large signal from blood if flow is predominantly in the
Z direction. After the 180.degree. pulse, there occurs an inflow of
fresh, unexcited nuclear spins from blood into the slab by the time
the 90.degree. pulse is applied to generate the signal. The extent
of flow over TI seconds is approximately equal to the average Z
velocity, V.sub.Z, times TI, and this value determines the usual
thickness of the excited slab. For example, if V.sub.Z equal 20
cm/second and TI equal 400 msc, then the excited slab should be
about 8 cm thick. If static material has a known and relatively
uniform T.sub.1 longitudinal relaxation time constant, then TI can
be selected so that the 90.degree. pulse occurs at the
magnetization "null" point of the static material when the
longitudinal relaxation crosses the zero point, as illustrated in
FIG. 4. However, because static material has different T.sub.1
values, some static material signals will remain. To the extent
that these remaining signals dominate the blood signals, it is
desirable to subtract them from the image. Several techniques are
available for removing the static material signals through
subtraction by acquiring a second image with the same static
material signals as in the first image but with having different
signals from blood.
For example, the same sequence as illustrated in FIG. 3 for
obtaining image 1 can be repeated but with the sequence gated to
the interval when the blood flow is relatively quiescent as
compared to the flow during image 1. Thus, there does not occur
much inflow of blood into the slab during TI. By selecting TI to be
the null point of blood, blood signals in image 2 are significantly
reduced. Static material signals remain the same in both images and
will subract out. Moreover, by keeping TI short, less T.sub.1
relaxation occurs and a larger difference signal can be
detected.
Alternatively, to avoid gating to different portions of the cardiac
cycle, and therefore to minimize the chances of motion related
artifacts, a change in the imaging sequence must be made in a way
that affects only flowing blood and not stationary material. One
such change is to alter the spatial selectivity of the inverting
180.degree. pulse for the second image. For example, by exciting
with a non-selective 180.degree. pulse, as illustrated in FIG. 5,
blood flowing into the slab defined by the 90.degree. pulse will
have experienced both excitations and, assuming TI corresponds to
the null point of blood or is short enough to avoid significant
relaxation, will yield a different blood signal. Thus, for image 1
the original selected 180.degree. pulse is still used. Hence, the
change in the imaging parameters that differentiate flowing blood
is the spatial selectivity of the 180.degree. pulse. This
subtraction approach allows for gating to the same portion of the
cardiac cycle for both images, thereby minimizing the possibility
of misregistration artifacts. Further minimization is possible by
interleaving the measurements for both images. Either phase
dependent subtraction or magnitude dependent subtraction can be
employed.
Other variations are possible with the latter method for
subtraction imaging. For example, it is possible to established
directional sensitivity by making the 180.degree. pulse for image 1
to be a "semi-selective" excitation whereby the slab and one
adjacent region are excited, as illustrated in FIGS. 6A and 6B for
the slab 21 with the excited region denoted by shading. In this
case, any material flowing in from the excited region will be the
same in both images and will therefore subtract out, leaving only
signals which have flowed into the slab from a region on the
opposite side of the slab as illustrated at 23. Alterntively, to
achieve the same directional sensitivity, the spatial selectivity
of the 180.degree. pulse for the second image can by adjusted to
excite the slab and the opposite side as illustrated in FIGS. 7A
and 7B. In this case, material flowing in from the unexcited side
can be large, but the same in both images, and will therefore
cancel out. In general, to eliminate material flowing in from a
particular side, the outer slab region from which it flows must
experience the same exitations for both images.
After the selected 90.degree. pulse, a spin echo may be created by
applying a 180.degree. pulse. This 180.degree. pulse can serve a
dual purpose to generate the spin echo in the slab and to invert
the out of slab region if the 180.degree. is non-selective. The two
images are derived by varying the spatial selectivity of this
180.degree. pulse. In the first case, the 180.degree. pulse is made
selective to the same slab and therefore the signal from the next
90.degree. pulse, 180.degree. pulse combination will contain a
large blood signal as unexcited spins flow into the slab. In the
second case, the 180.degree.0 pulse is made non-selective (or less
selective) creating a spin echo in the in-slab region and
simultaneously inverting the spins to the out-of slab region, thus
preparing the following 90.degree. pulse, 180.degree. pulse
combination which will yield a small signal as described before.
The selectivity of the 180.degree. pulse can be alternated between
measurements to interleave the two measurment sets. Gating is
required if the region of interest is pulsatile flow. This method
would be most useful if the heart rate is relatively fast so that
the interval between a 90.degree. pulse, 180.degree. pulse
combination is short enough to avoid a significant T.sub.1
relaxation of the blood component.
The basic sequence can be modified by having the 180.degree.
inversion pulse excite only the region adjacent to the slab excited
by the 90.degree. pulse. The inversion tags that adjacent region so
that any material from the tagged region flows into the slab in
time for the 90.degree. pulse to generate a signal. For the second
image, the 180.degree. pulse is simply not applied; instead, only
the 90.degree. pulse is used and the signal is quickly read out.
This second image will contain a relatively large static material
signal since the null point signal reduction does not apply in the
system but the two static material signals cancel. The flowing
material signal components will be different and thus will not
cancel.
Because projection imaging is of interest, the excited slab can be
imaged by 2-D techniques with spatial localization provided along
the z-direction and one perpendicular direction. Since the
field-of-view in the z-direction is limited by the thickness of the
slab, the imaging requirements for that direction are less severe
than normal. If spinwarp (2D-FT) imaging is employed, phase
encoding can be applied along the same direction as slice-selection
to reduce the number of measurements for the same resolution or to
increase resolution with same number of measurements. Also, in
contrast with other angiographic methods that rely on specific
gradient waveforms to produce flow sensitivity, the method in
accordance with the invention relies solely on time-of-flight
effects and can therefore incorporate a wider range of gradient
waveforms with which to image, including those that allow for fast
imaging. The imaging gradient waveforms are best chosen however to
be insensitive to motion (such as by making the first moments of
the gradient zero as taught in application Ser. No. 894,318, supra)
because the time-of-flight effects already provide the necessary
flow sensitivity. The velocity direction sensitized by these
methods is not solely along the z axis. The difference signal
arises in the regions that have been flowed into by fresh spins
during TI. Therefore, the regions visualized can be oriented in
various directions as long as the particular region is supplied by
spins from outside the slab within time TI. It is possible through
to augment this sensitivity by manipulating the gradients between
the first and second images. For example, if spinwarp imaging is
employed, the readout gradient waveform can be varied to sensitize
to flow in the direction perpendicular to z, as taught in
application Ser. No. 894,318, supra.
Another useful arrangement suitable for pulsatile flow regions is
to apply the 180.degree. inversion pulse just prior to the moment
of rapid flow (e.g. systole) to maximize the extent of inflow, and
the 90.degree. pulst during the period of quiescent flow
(e.g.diastole) to avoid artifacts and loss of signal due to
potential velocity-dependent phase shifts from the applied gradient
fields.
There has been describes an improved material imaging method and
apparatus in which the spatial selectivity of the excitation pulses
are controlled to remove static material and generate differential
signals from flowing blood. The invention has applicability to a
variety of vascular regions including coronary arties, and the
method is well suited for coronary artery imaging given the limited
field of views required. Moreover, embodiments of the invention
involving adjacent region excitation are particularly suitable
because the blood feeding the arteries comes from a large
well-defined region at the root of the aorta. The invention has
embodiments for pulsatile and non-pulsatile conditions and it can
be set up to provide one way directional sensitivity. While the
resulting image represents a limited field of view, image
acquisition time is reduced versus wide field of view imaging using
conventional techniques, and the procedure can be repeated for
other slabs to increase the field of view as required.
While the invention has been described with reference to specific
embodiments, the description is illustrative of the invention and
is not to be costrued as limiting the invention. Various
modifications and applications may occur to those skilled in the
art without departing from the true spirit and scope of the
invention as defined by the appended claims.
* * * * *